In many petrochemical and chemical processes, removal of contaminant provides process control and ensures compliance with environmental regulations. For example, syngas generated by gasification of coal or biomass, natural gases, gases generated from petroleum refining and processing can contain harmful and toxic gases such as H2S, COS, etc., which need be removed to control catalyst poisoning in the downstream processing and for regulatory compliance. In general, this removal is carried out by adsorbing the contaminant on the surface of sorbent materials in reactors, such as fluidized bed or moving bed reactors. In these reactors, the sorbent materials are normally in the form of pellets of sizes on the order of a few hundred microns to few thousand microns. To achieve an adequate removal capacity, sorbent materials with high surface area are normally used. For example, for the removal of H2S from a gas stream by adsorption, various porous materials such as activated carbon, modified clay, or modified zeolites have been used.
However, the available adsorption sites of the sorbent materials in a fluidized bed reactor or a fixed bed reactor are predominately located on the internal surfaces of the pellets. With this approach the gases must diffuse into the internal porosities of the pellets, which in turn, limits the removal rate. In addition, due to the interaction of gases with the internal surfaces and repeated expansion and contraction of the pellets in adsorption-regeneration cycles, the pellets become physically unstable. Due to this physical instability, the pellets can lose integrity, i.e., break apart mechanically, causing costly clean up and lost materials. To keep the operation running efficiently, new sorbent pellets must be added to the system and the disintegrated pellets must be removed from the reactors. Therefore, the current gas cleanup techniques suffer from this costly drawback arising from the aggregate forms of the absorbent materials used.
U.S. Pat. No. 5,494,880 describes the preparation of pellets with improved physical stability using a mixture of sorbent oxide, such as ZnO, a stabilizing amount of an inert refractory oxide and porous silica, held together with binders. However, the use of large amounts of materials other than the active sorbents can reduce the absorbing efficiency.
Recent developments in the removal of contaminant have involved utilizing nanostructures of absorbent materials. For example, Lee et al. (“Desulfurization Using ZnO Nanostructure Prepared by Matrix Assisted Method” Korean J. Chem. Eng., 26(2), 582-586) describes purported methods for removing H2S by a fixed-bed reactor containing nanosized ZnO, which are synthesized by the matrix-assisted method. Wang et al. (“Low-temperature H2S Removal From Gas Streams With SBA-15 Supported ZnO Nanoparticles,” Chem. Eng. J., 142 (2008) 48-55.) describes a purported mesoporous silica gel SBA-15 functionalized by ZnO nanoparticle for H2S removal from a gas stream. U.S. Patent Application Publication No. 20090114093 describes desulfurization of warm fuel gases by metal-based sorbents attached to a porous substrate. In addition, Sayyadnejad et al. (“Removal of Hydrogen Sulfide by Zinc Oxide Nanoparticles in Drilling Fluid” Int. J. Environ. Sci. Tech., 5(4), 565-569, 2008) describes purported removal of H2S gas in drilling fluid by ZnO nanoparticles prepared by spray pyrolysis. The disclosure of each of these publications is incorporated herein by reference in its entirety.
As such, there is a need for methods and apparatus that overcome the drawbacks of the existing technologies and remove contaminant in a more efficient and economical manner.